An ionization-type smoke detector is constituted so that both the inner and outer ionization chambers have a high resistance so that the insulation resistance between the respective electrodes must also be very high. For this reason an attempt has so far been made to increase the insulation resistance between the electrodes by corrugating the surface of the base on which each of the electrodes is mounted, thus lengthening the creepage distances between the electrodes.
Now an example of a conventional ionization-type smoke detector of this kind will be explained in reference to FIG. 1 of the attached drawings. In this drawing reference numeral 1 designates an insulating base the under surface of which is formed into concentric circles of corrugations. Below the surface of the insulating base 1 there are respectively supported an inner electrode 4 that is mounted together with a radioactive source 2 by a screw 3, an intermediate electrode 6 with a central opening 5, and a net- or grid-like air permeable outer electrode 7, whereby an inner ionization chamber 8 and an outer ionization chamber 9 are constituted by the inner and intermediate electrodes 4, 6, and intermediate and outer electrodes 6, 7, respectively. Further, in FIG. 1 the reference numeral 10 designates a circuit board of an insulating material which constitutes a circuit for detecting the electrical change in the outer ionization chamber 9, the outer periphery of the circuit board 10 being mounted on the upper part of the insulating base board 1 in contact therewith, and the respective electrodes 4, 6 and 7 are electrically connected to the circuit board 10. A rear cover 11 is mounted on the insulating base 1 at its upper rear side.
With such a constitution the creepage distances between the respective electrodes 4, 6, 7 become longer and the insulation resistances between the respective electrodes 4, 6 and 7 are respectively increased. However, like other detectors these ionization-type smoke detector are also now required to be of smaller size as the times demand. However, in this case there arises a defect that even if corrugations are formed on the under surface of the base 1 as shown in FIG. 1, the desired creepage distances cannot be obtained.